Where in the EU? The best I've found in Ireland requires adding €5 of prepaid credit at least every six months to prevent the number from being deactivated. As of yet, I have not managed to automate it.
Can't definitely find it, but in Orange Poland prepaid offer it costs ~$7 (29 PLN) to prolong the account for 365 days (regardless whether it is topped up or not), but it has to be activated manually, it is not the "normal" way to use a prepaid plan tbh.